The Sun as an X-ray Source

The Sun as seen in X-rays
(from the Yohkoh satellite)

The Sun has a surface temperature of approximately 6,000 Kelvin, or
around 10,000 degrees Fahrenheit. The solar surface emits most of its electromagnetic
radiation in the "visible spectrum," or the portion of the electromagnetic
spectrum we can see with our eyes. A 6,000 K star should be an
extraordinarily weak source of X-rays. However, we have known since the 1940s
that the Sun is, in fact, a very strong X-ray emitter. So what is going
on here?

The X-rays we detect from the Sun do not come from the Sun's
surface, but from the solar corona, which is the upper layer of the Sun's atmosphere.
Only very hot gases can emit X-rays, and the corona, at millions of
degrees, is hot enough to emit X-rays, while the much cooler surface of
the Sun
is not. Thus, the Sun's atmosphere is an excellent source of X-rays.

However, the discovery of the hot corona created a big problem for
astronomers and physicists.

The coronal heating problem

Simply stated, the problem is this: The corona is hot, the Sun is not
(relatively speaking). So how does a surface that is about 6,000 Kelvin heat
an atmosphere to a million Kelvin? The mechanism by which the solar
corona is heated is still not fully understood, though the problem has
been addressed by some of the sharpest minds in the field over the past
50 years. The mystery persists.

What we know

Click for larger image

There are cycles of behavior seen in the coronal structure with time
scales of 11 and 22 years. The 11 X-ray images of the Sun's atmosphere seen here
obtained between 1991 (at solar maximum) and 1995 (near solar
minimum), provide a dramatic
illustration of how the corona changes during the waning part of the
solar cycle. The X-ray Sun looks completely different from the Sun we
see in the sky.

An X-ray image reveals a bright glow for the corona and a
black disk for the
surface of the Sun. In the corona, the shape and character of the hot
gases are controlled by the solar magnetic
fields. There is a clear structure to the bright areas. Many
structures have a filamentary (or thread-like) appearance that seems
to link two regions. These filamentary structures are called
coronal loops. In general, the loops are hotter and denser than the
areas around them. For this reason, they appear brighter in the X-rays
spectrum.

As the solar activity cycle progresses from maximum to minimum, the
Sun's magnetic field changes from a complex structure to a simpler
configuration with fewer fields. Since the Sun's hot gases are
controlled by these fields, the X-ray images reflect this global
change, with an overall decrease in brightness by a factor of 100.

The hot corona and its structure are not some short-lived,
temporary phenomena. Although the corona is very dynamic with events
occurring on time scales of minutes (flares),
there are also long-lived structures that have time
scales of months. In fact, the general features of the X-ray-emitting
corona
have never ceased in the many years we have been observing them. This
video clip illustrates the variation of the X-ray
corona.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.